Phase 4: embed UI into kamado-api binary

The production image is now a single Go binary that serves both the
JSON/WebSocket API under /api and the Svelte dashboard at /.

- New internal/webui package embeds a dist/ subdir via //go:embed.
  A placeholder index.html is committed so `go build` works on a
  fresh checkout; anything else in dist/ is regenerated per build
  and gitignored.

- httpapi.Server.Handler mounts the embed.FS at / with SPA-style
  fallback: unknown non-/api paths serve index.html so client-side
  routes survive a reload. /api/* is carved out explicitly so POSTs
  or typos never accidentally shadow API semantics with HTML.

- api/Dockerfile grows a node:22 builder stage that runs
  `npm ci && npm run build`, and the Go stage copies ui/dist/ into
  internal/webui/dist/ before `go build`. Build context moves to
  the repo root (docker-compose + `make api` both updated) so the
  Dockerfile can see both api/ and ui/.

With this in place, `make up` brings the whole stack online at
http://localhost:8080 — API under /api, dashboard at /. The Vite
dev server on :5173 with the /api proxy is still available via
`make ui-dev` for hot-reload development.

.gitignore

@@ -1,6 +1,10 @@
 # Build artifacts
 build/
 dist/
+# ...but the Go embed target inside the api module keeps a tracked
+# placeholder index.html, so un-ignore that subtree. Its own nested
+# .gitignore re-excludes any build artifacts the Dockerfile drops in.
+!api/internal/webui/dist/
 *.o
 *.a
 *.so